Electrical and electronic components, e.g., thick and thin film hybrid circuits, tantalum capacitors, axial leaded film-foil capacitors, ceramic disk and tubular capacitors, electrolytic capacitors, radial film foil capacitors, resistors, stacked film capacitors, and related electrical and electronic components must be, in general, provided with a protective coating. Such a coating must provide ease in handling and be capable of providing the electrical and electronic component certain desired physical, thermal, electrical insulation and moisture resistance properties. The variety of different shapes of electrical and electronic components that are provided with a protective coating present certain unique problems. The composition used must not only provide the desired properties mentioned as a protective coating, it must also be one that can be applied to the component involved using the most suitable coating procedure. Moreover, as with any coating composition, it is desired that the protective coating be obtained in the most economical manner without compromise to the various properties desired.
In general, the protective barrier coating on an electrical or electronic component is provided by first coating the electrical and electronic component, or a portion thereof, with a resinous composition, and then heating the coated component at a suitable temperature and for a suitable length of time whereby the coating composition is cured and adheres to the underlying electrical or electronic component. The post curing step of the coating operation may be shortened in some cases by preheating the component before application of the resinous coating composition. The coating composition used may be, in some cases, either a solid, powdery composition, or, in other cases, one that is liquid, depending somewhat upon the substrate being coated. The use of such a coating composition and procedure is, however, attendant with certain disadvantages. In particular, where the resinous composition is solvent based, this may create environmental considerations. Moreover, such compositions sometimes result in the creation of bubbles and pin-holes in the coating layer created by entrapped air during the curing cycle. Furthermore, conventional thermal curing of resinous coating compositions is not only time consuming but also costly in terms of energy consumption, space, equipment, coating material usage and personnel.
Printing and screening inks are applied to a wide variety of substrates, e.g., metals, metal alloys, paper, thermoplastic and thermosetting resin layers, etc. Nevertheless, particular problems are presented when the substrate is heat sensitive. The ink composition applied must be capable of being cured under conditions not damaging to the substrate.
The ultraviolet curing of coating compositions has been known now for sometime. Exemplary of the prior art are U.S. Pat. Nos. 3,896,014; 4,349,605; and 4,390,401. U.S. Pat. No. 3,896,014 discloses liquid nail lacquer compositions which comprise as the essential components a polyene, a polythiol, a photocuring rate accelerator and, as disclosed by the patentee, a surfactant from a particular class. Among the preferred surfactants are sorbitan sesquioleate, sorbitan dioleate, sorbitan trioleate, pentaerythritol dioleate and pentaerythritol trioleate. Other surfactants found operative in the composition disclosed, but deemed by the patentee to be somewhat less efficient, include alkenyldimethylethyl ammonium bromide; di"coco"dimethyl ammonium chloride quaternary imidazolinium salt (from stearic acid); glyceryl monooleate; glyceryl dioleate; glyceryl trioleate and polyglyceril ester of oleic acid. Nevertheless, a host of other surfactants were discovered to be inoperative in the invention due to the fact that they were either insoluble in the composition, or they did not improve the wettability of the composition as it tended to "bead" when applied to the surface of the nail.
In U.S. Pat. No. 4,349,605, there are disclosed radiation curable polymeric compositions having flame retardant properties which comprise copolymers of ethylene and a comonomer which may be vinyl ester or an acrylate or a methacrylate, a hydrated inorganic filler, an alkoxy silane, and a lubricant comprising lauric acid and ethylene-bis-stearamide. The filler can, if desired, be silane treated, rather than adding the filler and silane separately to the composition. Such polymeric compositions are disclosed to be preferably cured by radiation means, although cross-linking of the polymers can also be achieved by chemical crosslinking or thermal crosslinking. According to the patentees, such polymeric compositions will hold very large amounts of filler and still provide high flexibility and a high degree of crosslinking. This is deemed by the patentees to be quite surprising as high flexibility and high crosslinking are generally incompatible, as are high crosslinking and high filler loading (which implies low crosslinkable polymer content). In compositions used for coating, e.g., extruding onto electrical wire and cables, best results, according to the patentees, are obtained when from 44 to 80% by weight of filler in the composition, or 22 to 59% volume of filler, preferably 50 to 57% by weight of filler in the composition, or 26 to 32% volume of filler are employed.
U.S. Pat. No. 4,390,401 discloses ultraviolet curable coating compositions which comprise polyunsaturated polyacrylates or methacrylates and as a wetting agent and adhestive promoter an acrylate or methacrylate of a polyalkylene oxide derivative of a mono-hydric alkyl/aryl phenol.
Others in addition to the patentees in U.S. Pat. No. 4,349,605 have disclosed using a pretreated filler in a polymeric composition. Thus, in U.S. Pat. No. 2,952,595, it is disclosed that vinyltriethoxysilane treated filler, e.g., hydrated silica, in amounts from about 11 to 34% by weight of the composition or 10 to 30% volume of filler, enhances the beneficial effect that radiation treatment has on the filled polyethylene. Thus, with such treated fillers the impact strength of the compositions was improved, and the brittle point was lowered, in addition to improved torsional hepteresis.
In Japanese 56147846 (Matsushita Electric Works) there is disclosed a photocurable polyester resin composition for the production of thick sheets which comprises unsaturated polyester resin, a photopolymerization initiator, and an inorganic filler, e.g., calcium carbonate, calcium silicate, titanium oxide, aluminum oxide, talc, clay, alumina, calcium hydroxide, and magnesium carbonate, coated with a surfactant. Those surfactants specifically disclosed are stearic acid, lauric acid, rosin, lignin, and cationic surfactant. According to the abstract, the treated filler is used at 16.6 to 80% by weight of the composition. The surface-treated filler is claimed to permit UV to penetrate the composition to deep inside, allowing production of thick sheets.
For some purposes, it would be desirable to obtain a composition which is sufficiently fluid to allow coating on a substrate yet has a volume % of filler which is sufficiently high to impart durability into the final formed coating. It is sometimes difficult to achieve the desired volume % of filler in a particular composition.